1. Field of the Invention
The present invention generally relates to optics. More specifically, the invention relates to systems and methods pertaining to optical waveguides.
2. Description of the Related Art
Optical communication systems are configured to propagate signals between various locations. Through at least a portion of such a communication system, the signals are provided as light that is propagated along an optical path. Numerous optical communication systems rely exclusively upon the transmission of single or lowest order mode light. For example, long distance and metro-range optical communication systems use single mode fiber. Single mode fiber offers a larger bandwidth than multi-mode fiber. This enables single mode fiber to propagate signals over greater distances than achievable with multi-mode fiber without the use of repeaters, for example.
Optical signals of a communication system using single mode fiber are often processed by various integrated optic devices. These integrated optic devices can include modulators, interferometers, distributed feedback elements, etc., all of which typically are based on planar waveguide technology.
At various locations along an optical path of a communication system, it may be desirable to reshape or scale the mode of light propagating along the optical path. For instance, mode typically is reshaped to satisfy mode-match requirements of optical components positioned along an optical path. For example, mode may be reshaped to accommodate a transition from an optical fiber to an integrated optic component. As used herein, the term “mode” refers to the spatial distribution of light relative to a cross-sectional area oriented normal to the optical path.
Transformation of modal properties through axial tapering of a dielectric waveguide is useful in several contexts. For example, mode-size transformation permits independent optimization of the mode size in different portions of the waveguide for effective input and output coupling. Mode-size transformation also can be used to obtain a narrow far-field of the outcoupled modes. An adiabatic taper from a single-mode to a multi-mode waveguide also permits robust coupling into the fundamental mode of a multi-mode waveguide. This is important in certain types of nonlinear waveguide devices that involve interactions between modes at widely separated wavelengths.
A prior art solution for mode matching uses an optical fiber and a micro-lens, e.g., a spherical lens or gradient index lens. The optical fiber and micro-lens allow for collection of light from the output component, e.g., an output fiber. The optical fiber and micro-lens also provide input coupling of light into the input component, e.g., an input fiber. Typical disadvantages of using such a solution include design difficulties in providing components that are configured to receive the input mode and provide an appropriately reshaped output mode.
Tapered optical fibers also have been used to reshape mode. A tapered optical fiber includes one or more tapered portions, i.e., portions that have cross-sectional areas that vary along their respective lengths. The tapered portions of these fibers typically are formed by controlled incremental heating. For instance, by heating a portion of a fiber, the fiber core tends to expand, thus resulting in a localized increase in cross-sectional area of the fiber. Simultaneous heating and pulling of the fiber results in a reduction of cladding and core dimensions. A potential disadvantage of using tapered optical fibers includes decreased mechanical strength of the fiber.
Additionally, integrated optic waveguides with continuous tapers and segmented tapers have been used for reshaping mode. As used herein, the term “segmented taper” refers to a waveguide taper that is composed of or divided into portions with different optical properties that are defined by dielectric boundaries. These dielectric boundaries are formed between the waveguide portions and portions of the substrate material, as viewed along the axis of light propagation. The term “continuous taper” refers to a waveguide taper in which light, upon its propagation, does not traverse dielectric boundaries between the waveguide portions and portions of the substrate material. Thus, in a waveguide with continuous taper, the taper changes its optical properties in a continuous and adiabatic fashion.
Waveguides with segmented tapers are capable of providing two-dimensional mode tapering. However, these waveguides typically are lossy due to the multiple dielectric boundaries formed between the segmented waveguide portions. In addition, precise control of segmentation is technologically involved. Therefore, it can be appreciated that there is a need for systems and methods that address these and/or other shortcomings of the prior art.